a) Field of the Invention
The present invention relates to a method of producing a support for a planographic printing plate, and, in particular, to a method of producing an aluminum support which is superior in an electrolytic ally graining property.
b) Related Art
Aluminum plate (including aluminum alloy plates) is used as a printing plate support, and particularly as offset printing plate support.
Generally, in order to use an aluminum plate as an offset printing plate support, it is necessary that the aluminum plate has the proper degree of adhesion with photosensitive material and moisture retention.
For this purpose, the surface of the aluminum plate must be uniformly and finely grained. Since this surface graining treatment exercises a remarkable influence on the printing performance and print durability of the plate material when offset printing is actually conducted after plate preparation, its quality is an important factor in the manufacture of plate material.
As a surface roughening method for the aluminum support used for the printing plate, the alternating current electrolytic graining method is commonly used, and as the electric current, ordinary sinewave current, or special alternating wave form current such as square wave are used. Surface graining treatment of the aluminum plate is conducted by means of alternating current using a suitable electrode of graphite, etc. as the opposite electrode, and the treatment is usually conducted once, but the pit depth obtained in this manner is generally shallow, and print durability has been inferior. As a result, numerous methods have been proposed for purposes of obtaining a suitable aluminum plate as a support for planographic printing plate which has a grain where the pits are uniform and fine with a depth which is deep compared with the diameter. As conventional examples of such methods, there is the surface graining method which uses special wave form for electrolytic treatment (Japanese Patent Unexamined Publication No. Sho-53-67507), the special ratio between electricity quantity of a positive period and that of negative period at the time of alternating electrolytic surface graining (Japanese Patent Unexamined Publication No. Sho-54-65607), special wave form (Japanese Patent Unexamined Publication No. Sho-55-25381), the combination of current density (Japanese Patent Unexamined Publication No. Sho-56-29699), etc.
In addition, combining with mechanical surface graining (Japanese Patent Unexamined Publication No. Sho-55-142695) is also known.
On the other hand, as the producing method for aluminum supports, an aluminum ingot (and alloy additive) is retained in a melted state and cast into a slab (400 to 600 mm thickness, 1000 to 2000 mm width, 2000 to 6000 mm length); after passing through a surface cutting process in which a planing machine is applied to the structurally impure parts of the slab surface to cut away the parts by 3 to 10 mm, a soaking treatment process is conducted in which the slab is maintained in a soaking pit at 480.degree. C. to 540.degree. C. for 6 to 12 hours for purposes of removing stress from the slab interior and making its structure uniform. Thereafter, hot rolling is conducted at 480.degree. C. to 540.degree. C. After the slab has been rolled to a thickness of 5 to 40 mm in hot rolling, cold rolling is conducted at room temperature to a predetermined thickness. Subsequently, annealing is conducted in order to make the structure uniform, and after the rolled structure has been homogenized, cold rolling is conducted to a predetermined thickness, and correction is conducted to obtain a plate with a good degree of flatness. The aluminum plates obtained in this manner have been used as a support for a planographic printing plate.
Yet, in the case of electrolytic surface graining treatment, the aluminum support which becomes the object of treatment is particularly easily influenced. In the case where the aluminum support is manufactured through processes of melting and holding, casting, surface cutting and soaking in this order or manner, there occurs scattering in the metal alloy components in the surface layer which lead to a drop in the yield of the planographic printing plate, even if heating and cooling are repeated and surface layers are scraped off in surface cutting.
To cope with this, U.S. Pat. No. 5,078,805 which corresponds to Japanese Patent Unexamined Publication No. Hei-3-79798 proposes a method capable of producing lithographic printing plates of superior quality and good yield by reducing the scattering in the material of the aluminum support and improving the yield of the electrolytic surface graining treatment.
In the former method of producing a support for planographic printing plate, casting and hot rolling processes are continuously conducted to form from a molten aluminum a thin plate of hot-rolled coil, and thereafter cold rolling, heat treating and correction are conducted to obtain an aluminum support. Then, the aluminum support thus obtained is subjected to the surface graining.
In the latter method of producing a support for planographic printing plate, the continuous casting and rolling processes are conducted using common twin rollers to directly form the plate from molten aluminum. Subsequently, cold rolling and heat treatment are conducted, and surface graining treatment is performed on the aluminum support which has undergone correction.
Yet, with regard to the former producing method, some components of the aluminum support causes scattering in the yield of the electrolytic surface graining treatment and in the surface graining aptitude. Further, even using the latter producing method, there is a defect that external appearance becomes poor due to the generation of streaks and creased/granular irregularities on the treated surface which has undergone surface graining.